The evolutionary chronicle of plant life on the Earth. Understanding of this history is largely based on knowledge of extant plants, but the fossil record is playing an increasingly important role in refining and illuminating this picture. Study of deoxyribonucleic acid (DNA) sequences has also been revolutionizing this process in recent years. The molecular data (largely in the form of DNA sequences from several genes) have been demonstrated to be highly correlated with other information. See Phylogeny
“Algae” was once a taxonomic designation uniting the lower photosynthetic organisms, but ultrastructural and molecular data have uncovered a bewildering diversity of species. Algae are now recognized as 10 divergent lineages on the tree of life that join organisms as distinct as bacteria and eukaryotic protozoans, ciliates, fungi, and embryophytes (including the land plants). In a biochemical context, the term “algae” defines species characterized by chlorophyll a photosynthesis (except Embryophyta); some of their descendants are heterotrophic (secondary chloroplast loss). Despite the variety of species it encompasses, the term “algae” also retains phylogenetic relevance. See Algae, Chlorophyll, Photosynthesis
Embryobionta, or embryophytes, are largely composed of the land plants that appear to have emerged 475 million years ago. The evidence indicates that land plants have not evolved from different groups of green algae (Chlorophyta) as suggested in the past, but instead share a common ancestor, which was a green alga. Land plants all have adaptations to the terrestrial environment, including an alternation of generations (sporophyte or diploid and gametophyte or haploid) with the sporophyte generation producing haploid spores that are capable of resisting desiccation and dispersing widely, a cuticle covering their outside surfaces, and separate male and female reproductive organs in the gametophyte stage. The life history strategies of land plants fall into two categories that do not reflect their phylogenetic relationships. The mosses, hornworts, and liverworts represent the first type, and they have expanded the haploid generation, upon which the sporophyte is dependent. Several recent analyses of DNA data as well as evidence from mitochondrial DNA structure have demonstrated that the liverworts alone are the remnants of the earliest land plants and that the mosses and hornworts are closer to the vascular plants (tracheophytes). The tracheophytes include a large number of extinct and relatively simple taxa, such as the rhiniophytes and horneophytes known only as Silurian and Devonian fossils. All tracheophytes are of the second category, and they have expanded the sporophyte generation. Among extant tracheophytes, the earliest branching are the lycopods or club mosses (Lycopodium and Selaginella), and there are still a diversity of other forms, including sphenophytes (horsetails, Equisetum) and ferns (a large and diverse group in which the positions of several families still are not clear). See Embryobionta
All seeds plants take the reduction of the gametophyte generation a step further and make it dependent on the sporophyte, typically hiding it within reproductive structures, which are either cones or flowers. The first seed plants originated at least by the Devonian, and they are known to have a great diversity of extinct forms, including the seed ferns. There are two groups of extant seed-bearing plants, gymnosperms and angiosperms. In the gymnosperms, the seeds are not enclosed within tissue derived from the parent plant. There are four distinct groups of extant gymnosperms, often recognized as classes: Cycadopsida, Gnetopsida, Ginkgoopsida, and Pinopsida.
The angiosperms (also flowering plants or Magnoliopsida) are the dominant terrestrial plants, although the algae collectively must still be acknowledged as the most important in the maintenance of the Earth's ecological balance (fixation of carbon dioxide and production of oxygen). In angiosperms the seeds are covered by protective tissues derived from the parental plant. There are no generally accepted angiospermous fossils older than 120 million years, but the lineage is clearly much older based on DNA clocks and other circumstantial lines of evidence such as their current geographic distributions.
Traditionally the angiosperms have been divided into two groups, monocotyledons (monocots) and dicotyledons (dicots), based on the number of seed leaves. However, DNA sequence data have demonstrated that, although there are two groups, these are characterized by fundamentally different pollen organization, such that the monocots share with a group of dicots pollen with one pore whereas the rest of the dicots have pollen with three (or more) pores.